S = (50  -49)(50  +49) + (50 - 48)(50 + 48) + .... + (50 + 48)(50  -48)

S = 50² - 49² + 50² - 48² + ..... + 50² - 48² + 50² - 49

S = 99  . 50² - 2.(0² + 1²  +.... + 49²)

a) A=\(\sqrt{25}.\sqrt{3}+\sqrt{16}.\sqrt{3}-\sqrt{100}.\sqrt{3}\)

A= \(5\sqrt{3}+4\sqrt{3}-10\sqrt{3}\)

A=\(-\sqrt{3}\)

b) \(\sqrt{\left(x+1\right)^2}\) =2

\(\Leftrightarrow\) \(\left|x+1\right|=2\)

\(\Leftrightarrow\) \(\left\{{}\begin{cases}x+1=-2\\x+1=2\end{cases}}\) ( Cái này là do lỗi của hệ thống, mink ko biết cách sửa. mong thông cảm để mình ghi lại cho bạn dễ nhìn

\(\Leftrightarrow\)x-1=2 với x>0

x-1=-2 với x<0

\(\Leftrightarrow\)x=1 hoặc x=-3

Choose the letter A, B, C, D for each gap in the following sentence.

Lan promises she _________ her best in learning English.

A. try

B. will try

C. trying

D. tries

Mai Thúc Loan được nhân dân tôn xưng là

A. Tiền Ngô Vương

B. Mai Hắc Đế

C. Hoài Vũ Vương

D. Dạ Trạch Vương

Mark the letter A, B, C, or D to indicate the correct response to each of the following exchanges. Two friends Nam and Lan are talking about the topic of global warming.

Lan: What solution to air pollution can you suggest? - Nam: ____

A. Air pollution is really serious these days.

B. Smoke from factory chimneys pollutes the air.

C. Air pollution causes acid rain.

D. People should use public means of transport.

Read the passage, then answer the questions below (2.5pts)

Are you looking for a cheap, clean, effective source of power that doesn't cause pollution or waste natural resources? Look no further than solar energy from the Sun.

At present, most of our electricity comes from the use of coal and gas, oil or nuclear power. This power could be provided by the Sun. One percent of the solar energy that reaches the Earth is enough to provide power for the total population.

Many countries are already using solar energy. Solar panels are placed on the roofs of a house and the Sun's energy is used to heat water. The energy can be stored for a number of days, so on cloudy days you can use solar energy, too.

Sweden has an advanced solar energy program. There, all buildings will be heated by solar energy and cars will use solar power instead of gas by the year 2015.

Questions: How long can the solar energy be stored?